Head and neck cancers annually account for 3.2% (39,750) of all new cancers and 2.2% (12,460) of all cancer deaths in the US. Approximately 45% of these patients will relapse after primary therapy with surgery and radiation with or without chemotherapy. Recurrences in patients who previously received chemotherapy tend to be more aggressive, with one study reporting a median survival of 107 days. In this proposal we address the urgent need for a minimally invasive, non-toxic, cost-effective, local modality for treating recurrent head and neck cancers with minimal treatment-associated morbidity, which will allow the patient to function as normally as possible in their remaining life-time. We believe this can be achieved by combining Photodynamic therapy (PDT) with photoacoustic lifetime imaging (PALI), a newly developed technique for tissue oxygen imaging. PDT is a light-activated local cancer treatment. Extensive studies during the last 2 decades have established PDT as effective in the treatment of head and neck cancers. However, tissue oxygenation and light penetration strongly affect the efficacy of PDT. We hypothesize that the ability to measure tissue oxygen using PALI, and augmenting its level in the tumor environment will improve the treatment, its outcome and cure rates. Moreover, PALI images will provide a real-time assessment of light penetration to the treated site and the possibility of personalized optimization of PDT light dose. This R01 proposal is based on the results of an on-going R21 project that show feasibility of imaging tissue oxygen in small animals. We will first conduct a comprehensive investigation of the PALI technique in order to accurately characterize its precision and regions of applicability with respect to environmental parameters such as pH and temperature (Aim 1). The scope of the PALI application in an in vivo environment will be tested on small animals (Aim 2). Based on the experimental outcomes, we will develop a portable clinical system for PALI guided PDT treatment (Aim 3). The system will be evaluated in large animal head and neck cancer models. A more extensive study will then be conducted to test the hypothesis that PALI feedback can guide parameter modification (including gas mixture breathing and light dose) on a case-by-case basis, to optimize the efficacy of PDT treatment (Aim 4). Ultimately, we expect the technique to be translated to clinics, significantly improving treatment outcomes of head and neck cancer patients. The results of this project could also drive the development of PALI for other clinical applications, such as guiding chemotherapy and radiotherapy.